LIFE WOULD be a lot more pleasant if we didn't have to live it in such chronological order. It's so unkind. You start off with all that flexibility, elasticity, energy, brain speed and voracious metabolism and just when you start to realise how good it was, it disappears into the halcyon mist that was your youth.

Do I sound gloomy? Maybe it is because I have reached one of the pointers on life's ageing map: the late twenties crisis. An ageing pointer is that moment when, after years of looking blithely in the mirror, you catch a sudden objective glimpse of yourself and notice change. The first such age shock often happens at 23, when the mirror says, "God, I'm not 18 any more." As you get older the mirror asserts itself more and more often.

The next marker is late twenties angst, when physical changes are compounded by a sense of worry about what hasn't happened in your life. "I'm nearly 30 and I am no more sorted out than I was at 22," says Ruth, 29. "I think I would be more stoic if I weren't single. But I noticed my first cellulite three months ago, something I prided myself on not having."

The challenge is to look better than your contemporaries. Ruth should actually feel pleased that cellulite left her alone for that long. Most of us have some cellulite by 25. It is very useful to know what's normal in the ageing process. You might just find you have been over-critical of yourself.

1) Wrinkles

Surgeon Maurizio Viel who, with his twin brother, Roberto, owns the London Centre for Aesthetic Surgery in Harley Street, says fine lines start appearing from 25. "At this stage they can be covered by good make-up. By 29 the fat on your face is beginning to redistribute." Don't con yourself that your cheek bones are more pronounced - everything is beginning to slump downwards.

Some doctors say a wrinkled skin isn't inevitable genetically until our sixties. Environmental factors are more important before that. Sun damage causes up to 90 per cent of visible skin damage. And smoking speeds up the wrinkling process by up to ten years.

Abusing your skin won't show much in your twenties. But soon, says Viel, you can tell the water drinkers from the sun-soaking chain smokers. "By 35 there can be as much as 10 years' difference just from different lifestyles."

2) Fitness

30 is a milestone for athletic performance, when the body loses the edge of its speed. Fortunately, since most of us fail to reach our speed peak anyway, this loss passes unnoticed. Mid-thirties is a crucial time for cardio-vascular fitness. It starts in earnest now and will drop by up to 40 per cent by 65.

3) Grey Hair

34 is the average age that a white person starts developing grey hair. Black people typically start about ten years later. By 50, 50 per cent of people are 50 per cent grey. So if you're 40 and grey-free start feeling smug.

4) Mental Performance

"From the age of 35 short term memory is not as good as it was," says Ronald Klatz, President of the American Academy of Anti-Ageing Medicine in Chicago. "Our mental lists fail, we can't hold as many phone numbers in our heads." But surely after 35 the brain is better in other ways? "Not really," says Dr Klatz, "just more cunning at hiding its inefficiencies."

5) Weight

Metabolism reaches a peak in adolescence. To maintain weight, says Dr Klatz, eat 200 calories less each decade. But it is within the normal range to gain about five pounds with each decade after 20. By 40 there is a dilemma: skinny looks cooler, plumper can look nicer - and younger. "My mother always said women aged in one of two ways," says Meg, 43. "Prune or plum. I am going down the prune route. It means I can still wear fashionable clothes, but I look older than some of my fatter friends close-up, but younger further away."

Sadly, there is only space to cover the early ageing process today. In part two, we will examine muscle wastage, sexual decline, senility and death. Brace yourself.